Audio signals are normally amplified using class D amplifiers, inter alia. In line with the underlying principle, a reference signal is first compared with a signal fed back from the output and a corresponding error signal is output. This signal is processed on a pulse width modulation basis using a sawtooth signal and is passed to an output amplifier stage. In this case, the output stage is operated so as to switch at a particular duty ratio. To maintain a flow of current at the output, a freewheeling diode and an inductor are provided at the output. It is thus possible to provide a constant output current at the output.
FIG. 1 in the “Journal of the Audio Engineering Society”, Audio Engineering Society, New York, USA, vol. 39, No. 9, Sep. 1, 1991, pages 650, 662 shows an amplifier circuit with a differential amplifier whose output side is connected to a first input on a comparator. A second input on the comparator is supplied with a sawtooth-waveform signal. The output side of the comparator is connected to an output circuit which is pulse width modulated.
Printed document EP 0503571 A1 likewise shows a pulse width modulated amplifier circuit whose signal input is connected to a comparator. A second input on the comparator is supplied with a sawtooth-waveform signal. The output of the comparator is connected to an output stage.
However, such amplifiers have relatively poor properties in terms of the power supply rejection ratio (PSRR). If the low-level signal response of a circuit of this type is considered, the ratio of the output voltage to the input voltage is
            Vout      Vin        =          d              1        +                              s            2                    ⁢          LC                      ,where L and C are the values of an LC filter at the output and d is the duty ratio of the pulse width modulation. The gain of the transfer function Vout/Vin is accordingly proportional to the duty ratio d, which may be between 0 and 1. However, this term determines the denominator of the formula for describing the power supply rejection ratio. Accordingly, fluctuations in the supply voltage or radio-frequency interference components in the supply voltage result in relatively severe unwanted effects on the output signal from the amplifier circuit.
A further problem of the principle described is the unwanted convolution of signals. If the supply voltage for the output stage behaves like a relatively low-frequency sinusoidal oscillation but the useful signal is likewise a (higher-frequency) sinusoidal oscillation, then the low-level signal gain also varies sinusoidally. The resultant harmonics are at the summed frequency and the differential frequency between the frequencies of the two signals at an amplitude which corresponds to half of the product of the amplitudes of the two signals. The problem is of great significance particularly because the interference may be at frequencies below the cut-off frequency of the low-pass filter at the output and is therefore not filtered out.
The relatively poor power supply rejection ratio is normally countered by increasing the signal gain. This increases the power consumption, however.
The convolution problems described may be reduced by reducing the noise components and interference components on the supply voltage, for example by using a linear controller. This severely reduces the efficiency of the amplifier, however.